# Math Help - one to one function with unknown constant!

1. ## one to one function with unknown constant!

This problem is driving me crazy.

Let f(x)= 3x^3 + kx^2 +5x, k is a constant. For what values of k is f one to one?

I tried using b^2 -4ac from the Quadratic Formula but I didn't get the same answer as the answer choices...
a. |k| <= 3 root5
b. 5/9 <= k
c. |k|<= 6 root5
d. k <= 5/9
e. 3 root5 <= |k|

Consider the derivative $f'(x) = 9x^2 + 2kx + 5$.